Serine proteases are a subgroup of a diverse class of enzymes having a wide range of specificities and biological functions (See e.g., Stroud, Sci. Amer., 131:74-88 [1974]). Despite their functional diversity, the catalytic machinery of serine proteases is represented by at least two genetically distinct families of enzymes: 1) the subtilisins; and 2) the mammalian chymotrypsin-related and homologous bacterial serine proteases (e.g., trypsins). These two families of serine proteases show remarkably similar mechanisms of catalysis (See e.g., Kraut, Ann. Rev. Biochem., 46:331-358 [1977]). Furthermore, although the primary structure is unrelated, the tertiary structure of these two enzyme families brings together a conserved catalytic triad of amino acids consisting of serine, histidine and aspartate.
In contrast, the subtilisins and chymotrypsin-related serine proteases both have a catalytic triad comprising aspartate, histidine and serine. In the subtilisin-related proteases the relative order of these amino acids, reading from the amino to carboxy terminus, is aspartate-histidine-serine. However, in the chymotrypsin-related proteases, the relative order is histidine-aspartate-serine. Much research has been conducted on the subtilisins, due largely to their usefulness in cleaning and feed applications. Additional work has been focused on the adverse environmental conditions (e.g., exposure to oxidative agents, chelating agents, extremes of temperature and/or pH) which can adversely impact the functionality of these enzymes in various applications. Nonetheless, there remains a need in the art for enzyme systems that are able to resist these adverse conditions and retain or have improved activity over those currently known in the art.